As representative means for measuring a film thickness of a laminated wafer, there has been known a film thickness measurement method which uses FTIR (Fourier Transform Infrared) method (Patent Literature 1). A FTIR measurement device described in Patent Literature 1 includes: an infrared light source 1 capable of emitting infrared rays; a beam splitter 2 which splits a beam of light in two such that a portion of the incident light is allowed to pass through the beam splitter 2 and a remaining portion of the light is reflected; a scanning mirror 3 movable in an optical axis direction; a fixed mirror 4; and a detector 5. The FTIR measurement device detects an interferogram by measuring reflection light by using the detector 5, the reflection light being generated when a wafer is irradiated with an infrared ray, and calculates a film thickness based on the detected interferogram.
An edge part of a laminated wafer is not flat and has a shape with a change in angle. Accordingly, when the above-mentioned FTIR measurement device is applied to the measurement of a thickness of an edge part of a laminated wafer, an interferogram is not detected thus giving rise to a drawback that a thickness of an adhesive agent which forms the laminated wafer cannot be accurately obtained.
Besides the above, as a method for measuring a thickness of an adhesive agent of an edge part of a laminated wafer, a method is used where a laminated wafer is cut and a cross-section of the laminated wafer is observed. However, this method adopts destructive inspection and hence, processing of a wafer requires a considerable time thus giving rise to a drawback that a yield is lowered.